Multistaged polymers containing a hydrophilic core polymer encapsulated by a hydrophobic shell polymer are known. It was, however, believed that a shell polymer of extremely non-polar or low-polar hydrophobic monomers ("hydrophobic shell"), such as styrene, .alpha.-methyl styrene, vinyl toluene, ethylene, vinyl chloride and vinylidene chloride, could not be formed directly on a core containing a high level of hydrophilic monomers ("hydrophilic core") without either:
(1) copolymerizing the extremely non-polar or low-polar hydrophobic monomers with vinyl acetate, acrylonitrile or methacrylamide; or PA1 (2) first forming a tiecoat on the hydrophilic core. PA1 (1) emulsion polymerizing a hydrophilic core polymer from about 5% by weight to about 100% by weight, based on the total weight of the core polymer, of a hydrophilic monoethylenically unsaturated monomer and from 0% by weight to about 95% by weight, based on the total weight of the core polymer, of at least one nonionic monoethylenically unsaturated monomer; PA1 (2) emulsion polymerizing, in the presence of the core polymer, at least one shell polymer from about 90% by weight to about 99.9% by weight, based on the total weight of shell polymer, of at least one nonionic monoethylenically Unsaturated monomer and from about 0.1% by weight to about 10% by weight, based on the total weight of the shell polymer, of an acid-functional monoethylenically unsaturated monomer, PA1 wherein the acid-functional monoethylenically unsaturated monomer is added to the polymerization of the shell polymer over 100% of the total shell monomer feed when the particle size of the core polymer is from about 130 nm to about: 2.0 microns and over the first 50% of the total shell monomer feed when the particle size of the core polymer is less than about 130 nm. PA1 (1) emulsion polymerizing a hydrophilic core polymer; and PA1 (2) emulsion polymerizing, in the presence of the hydrophilic core polymer, at least one hydrophobic shell polymer to completely encapsulate the core. The crux of the invention is the manner and timing of the addition of the acid-functional monoethylenically unsaturated monomer in the polymerization of the hydrophobic shell polymer. When the particle size of the core polymer is from about 130 nm to about 2.0 microns, the acid-functional monoethylenically unsaturated monomer used to make the shell may be added over 100% of the shell monomer feed, based on the total weight of the shell monomer feed, preferably over the first 50% of the feed, more preferably over the first 25% of the feed and most preferably over the first 10% of the feed. When the particle size of the core polymer is less than about 130 nm, the acid-functional monoethylenically unsaturated monomer may be added over the first 50% of the feed, more preferably over the first 25% of the feed and most preferably over the first 10% of the feed. PA1 (1) stabilizing of the final sequential polymer dispersion; PA1 (2) assuring permeability of the hydrophobic shell polymer to a swellant for the hydrophilic core polymer; and PA1 (3) compatibilizing the hydrophobic shell polymer with the hydrophilic core polymer so that the core may be fully encapsulated with shell.
The tiecoat (referred to in some of the prior patents as "the first stage of sheath formation") was generally an acrylic polymer which compatibilized the hydrophilic core polymer with the one or more hydrophobic shell polymers, particularly for a hydrophilic core polymer having a particle size diameter of less than about 280 nanometers (nm).